1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and more particularly to homogenization of a potential of an electrode common to plural pixels arrayed in a display area among electrodes of organic light emitting elements.
2. Description of the Prior Art(s)
The organic EL display device has a structure in which a TFT substrate is bonded to a counter substrate. FIG. 7 is a schematically partial cross-sectional view of a display area of a TFT substrate 4 in a related art organic EL display device 2, which illustrates a vertical cross-section along a horizontal direction of the display area. The TFT substrate 4 is configured, for example, so that a circuit layer 8, an insulating film 10, a bank 12, an organic light emitting diode (OLED) 14, and a sealing film 16 are laminated on a glass substrate 6.
Plural pixels are two-dimensionally arrayed in a display area. In an organic EL display device that conducts color display, each of the pixels includes plural sub-pixels that emit lights of different colors. For example, each of the pixels can be configured by an R sub-pixel that emits light of red (R), a G sub-pixel that emits light of green (G), and a B sub-pixel that emits light of blue (B). One pixel is also configured by four sub-pixels adding a W sub-pixel that emits light of white (W) to those sub-pixels.
The OLED 14 is formed in an opening area surrounded by the bank 12 (partition) for each sub-pixel, and includes a lower electrode 20, an organic layer 22 (organic material layer), and an upper electrode 24. The lower electrode 20 and the upper electrode 24 configure an anode electrode (anode) and a cathode electrode (cathode) of the OLED, respectively, and light emission in the organic layer 22 is controlled according to an electric signal applied between those electrodes. The upper electrode 24 is basically shared by the pixels in the overall display area, connected to a feeder line on an edge of the display area, and applied with a common voltage from a drive unit. On the other hand, the lower electrode 20 is formed for each of the pixels, and supplied with a current corresponding to a video signal from the drive unit through a pixel circuit of each pixel and a drive power supply line which are formed in the circuit layer 8.
The bank 12 functions as a partition for partitioning the OLED, particularly the organic layer 22 for each of the sub-pixels. Also, the bank 12 is formed of an insulating layer, and prevents short-circuiting between the respective lower electrodes 20 formed in every opening area.
The upper electrode 24 and the lower electrode 20 are made of a transparent conductive material such as IZO (indium zinc oxide) or ITO (indium tin oxide), for example. In particular, in the organic EL display device of a top emission type, it is desirable that that transmittance of light in the upper electrode 24 is higher from the viewpoint of a reduction in the power consumption. In this example, if the transparent conductive material is thinned, the transmittance of the upper electrode 24 increases. On the other hand, the electric conductivity of the upper electrode 24 decreases, and a voltage drop caused by a current that flows in the OLED increases. Specifically, a potential difference between a position closer to the feeder line and a position farther from the feeder line in the upper electrode 24 becomes larger. As a result, shading (luminance gradient) in which a current density of the OLED decreases more to reduce the brightness more at a position farther from the feeder line occurs. That is, there arises such a problem that the light emission brightness does not become uniform, and display unevenness is generated within the display area. This problem becomes more remarkable as an area of a display panel is larger.
Under the circumstances, a configuration in which a feeder line that assists conduction is made of a material low in resistance value, and the upper electrode 24 and the feeder line are connected to each other through a contact hole formed in the bank 12 to substantially decrease the resistance of the upper electrode 24 has been proposed (JP 2009-199868 A).
FIGS. 8 and 9 are plan views of a part of a display area of the organic EL display device in which a contact hole 32 that connects the upper electrode 24 and a feeder line 30 is formed in the bank 12. In this example, each of pixels 34 includes four sub-pixels 36 of RGBW. FIG. 8 illustrates a case in which the contact hole 32 is arranged in the bank 12 between the pixels 34 adjacent to each other in a horizontal direction, and FIG. 9 illustrates a case in which the contact hole 32 is arranged in the bank 12 between the pixels 34 adjacent to each other in a vertical direction.